# When did the error function get its modern definition?

I am currently writing an essay on the error function and after researching its historical origin, I found out who first defined it: J.W.L. Glaisher. But his definition is different from today's form. His definition: $$\int_x^∞e^{-x^2}dx = \operatorname{erf}(x)$$ and today: $$\frac{2}{\sqrt{\pi}}\int_0^xe^{-x^2}dx$$. I would like to know when they changed it and what the reason was.

Harold Jeffreys's 1927 book Operational Methods in Mathematical Physics has a 2-page note titled "On the notation for the error function or probability integral" on pages 94-95, with 9 footnotes, addressing your question. (The link I give has a free pdf download option.) He seems to say that Jeans introduced the $$\frac2{\sqrt\pi}$$ factor, in his 1921 book The Dynamical Theory of Gases.
A partial resolution of the inconsistency of names is to see that where Glaisher, Whittaker and Watson, etc. use $$\operatorname{Erf}$$ and $$\operatorname{Erfc}$$ for the unscaled functions, A&S, Wikipedia, DLMF, etc. use $$\operatorname{erf}$$ and $$\operatorname{erfc}$$ for the scaled ones. But Jeffreys uses $$\operatorname{Erf}$$ for the scaled version, so the inconsistency is not eliminated.
• @njuffa I meant, J&E. The version at babel.hathitrust.org/cgi/… shows J&E using $\Phi(x)$ to denote the modern $\operatorname{erf}$. That is, J&E did not use the erf notation. I am sure Jeffries's German was better than mine, and J&E is not at all hard to read. Commented Feb 7, 2023 at 21:35
• @njuffa J&E's $\Phi$ is not the modern $\Phi$ because there is a factor of $\sqrt 2$ applied to the argument. The one is the indefinite integral of $\exp(-x^2)$ and the other is that of $\exp(-x^2/2)$. Commented Feb 7, 2023 at 22:04